In the field of packaging machines problems due to the need for continuous changes of the speed of the plastic film subjected to welding are known. Said speed changes are due to the unavoidable irregularities of supply to the machine of products to be packaged, irregularities which require acceleration and slowing of the plastic film to avoid production of empty packages or with the surface printing off center.
The biggest problem is due to the fact that the films of heat-welding material unfortunately have a limited field of weldability depending on their speed of travel and the temperature of the heated welding elements. Given the frequency with which the speed changes, and the thermal inertia of the heating elements, it is not possible to change quickly enough the temperature of said welding elements and hence it happens that excessive slowing of the film causes its complete melting and burning while excessive speed causes defective welding or lack of welding.
In automated production lines where the range of strip speeds is not guaranteed inside the limits of weldability of the heat-welding materials it is therefore necessary to use cold welding film, which has much higher cost than normal heat-welding film.
Moreover, even in the case of systems with speeds usually held within the limits allowing the use of heat-welding materials, it happens occasionally that the system, for various and unforeseeable reasons, must suddenly slow down more than normally, which causes burning of the film and production of a certain number of defective packages for the entire duration of the slow-down.